Fix LIGHTEN. Close #2723 (#2744)

### What problem does this PR solve?

Fix LIGHTEN
#2726 
#2723

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

api/settings.py

 SERVER_MODULE = "rag_flow_server.py"
 TEMP_DIRECTORY = os.path.join(get_project_base_directory(), "temp")
 RAG_FLOW_CONF_PATH = os.path.join(get_project_base_directory(), "conf")
-LIGHTEN = os.environ.get('LIGHTEN')
+LIGHTEN = int(os.environ.get('LIGHTEN', "0"))
 
 SUBPROCESS_STD_LOG_NAME = "std.log"